We have used quantitative ligand binding studies to characterize the multiple types and subtypes of opioid receptors in rat brain, bovine adrenal medulla, cultured chromaffin cells, and other peripheral tissues. We have demonstrated the existence of the previously known mu, delta and kappa receptors, and in addition the existence of mu-l and mu-2 subtypes, and of kappa-l, kappa-2, kappa-3 subtypes with distinctive pharmacological profiles. This has been made possible by use of computer simulation to optimize experimental design for experiments involving the simultaneous presence of multiple ligands, the combined use of pharmacological blockade and computer modelling of multiple data sets simultaneously with appropriate constraints.